1. Field of the Invention
The present invention relates to a leakage flux flaw detecting method in which a magnetic field is applied to a ferromagnetic substance to detect magnetic flux leaking from the ferromagnetic substance, by which flaws such as inclusions present in the ferromagnetic substance are detected, and a method for manufacturing a hot rolled steel sheet or a descaled steel sheet using the leakage flux flaw detecting method.
2. Description of Related Arts
As a method for detecting flaws such as inclusions present in a ferromagnetic substance such as a steel sheet, a leakage flux flaw detecting method has widely been used.
As one example, FIG. 1 shows a configuration of a magnetic flaw detector using a magnetic sensor incorporated in a steel sheet inspection line. A magnetic flaw detector 4 is disposed along a transferring path for a steel sheet 1 (ferromagnetic substance) transferred on a product inspection line at a substantially constant speed V by transferring rollers 2 and 3. The magnetic flaw detector 4 includes a magnetizer 5 for magnetizing the running steel sheet 1, a magnetic sensor 6 disposed at a position opposed to the magnetizer 5 so that the steel sheet 1 passes therebetween, and a signal processing unit 7 for processing output signals sent from the magnetic sensor 6.
If a flaw 8 exists in the steel sheet 1 when the steel sheet 1 is magnetized by the magnetizer 5, magnetic flux passing through the interior of the steel sheet 1 is disturbed by the flaw 8, and some of the magnetic flux leaks to the outside of the steel sheet 1. This leakage flux is detected by the magnetic sensor 6, and an output signal sent from the magnetic sensor 6 is processed, by which the flaw 8 can be detected. Also, since the intensity of the leakage flux depends on the size of the flaw 8, the size of the flaw 8 can also be evaluated by the output signal level of the magnetic sensor 6.
On the other hand, the leakage flux detected by the magnetic sensor includes, in addition to the leakage flux caused by a flaw, disturbance of leakage flux caused by nonuniformity of local magnetic characteristics of the steel sheet (nonuniformity of thickness of oxide scale, irregularities of oxide scale/ground steel interface, and the like) or surface roughness. Such disturbance of magnetic flux is unnecessary magnetic flux or noise from the viewpoint of flaw detection.
In order to exclude the influence of such noise, a method, described below, is sometimes used which uses different frequency characteristics of the output signal caused by a flaw (hereinafter referred to as a flaw signal) and the output signal caused by noise (hereinafter referred to as a noise signal).
FIG. 2 shows an example of frequency characteristics of flaw signal and noise signal measured when a steel sheet runs at a constant speed. Generally, as shown in FIG. 2, the flaw signal has higher frequency distribution than the noise signal. Therefore, the flaw signal can be highlighted relatively and extracted by incorporating a bypass filter with a cut-off frequency f in the signal processing unit. Also, a method using a filter having a proper constant to enhance the flaw detecting ability in the leakage flux flaw detecting method has been disclosed in Unexamined Japanese Utility Model Publication No. 61-119760.
However, as shown in FIG. 2, the frequency characteristics of the flaw signal and the noise signal overlap partially. Therefore, in the case where the flaw is minute or the noise is much, even if the bypass filter is provided to discriminate the flaw signal from the noise signal by means of frequency, it is difficult to exclude the influence of noise to a degree such that a flaw can be detected with high accuracy.
An object of the present invention is to provide a leakage flux flaw detecting method that is capable of detecting flaws even if the flaws are minute or unnecessary magnetic flux or noise is much and a method for manufacturing a hot rolled steel sheet using the leakage flux flaw detecting method.
The above object is achieved by a leakage flux flaw detecting method comprising the steps of: magnetizing a ferromagnetic substance successively to a plurality of different intensities of magnetization; detecting magnetic flux leaking from the same position of the ferromagnetic substance having been magnetized to each of the intensities of magnetization by using a magnetic sensor; and processing output signals of the magnetic sensor corresponding to each of the intensities of magnetization so that a signal caused by a flaw in the ferromagnetic substance is highlighted. Also, a leakage flux flaw detecting method can be used which comprising the steps of: magnetizing a ferromagnetic substance by one or a plurality of magnetizers; detecting magnetic flux leaking from the same position of the ferromagnetic substance having been magnetized successively by a plurality of magnetic sensors provided at positions where the intensity of magnetization is different while the ferromagnetic substance is moved along the magnetic sensors; and processing output signals of the magnetic sensors so that a signal caused by a flaw in the ferromagnetic substance is highlighted.
Also, a hot rolled steel sheet covered with oxide scales or a descaled hot rolled steel sheet in which accurate flaw information such as flaw location and density is known can be manufactured by using a method for manufacturing a hot rolled steel sheet, comprising the steps of: performing hot rolling; detecting flaws in the hot rolled steel sheet by using the above described leakage flux flaw detecting method in accordance with the present invention; and determining the information of the detected flaws.